The technical field of this invention is corneal surgery and, in particular, the invention relates to systems and methods of compensating for presbyopia by reprofiling the cornea of the eye.
As individuals age, the human eye loses its ability to focus on nearby objects. Between the fourth and fifth decades of life, the condition often becomes so pronounced that simple tasks, such as reading are difficult. This condition, known as presbyopia, is due to a progressive loss in the elasticity of the lens of the eye, such that the ciliary muscles which normally force the lens in a rounded shape to accommodate near objects can no longer exert the necessary changes in the lens' shape.
The conventional optometric solution to the problem of presbyopia is a prescription of reading glasses or, for individuals who already require glasses to correct other refractive errors such as myopia or astigmatism, a prescription of bifocal or multifocal glasses.
Recently, it has been demonstrated that changes in the refractive power of the eye can be achieved by laser ablation of the corneal surface. Such procedures, known as photorefractive keratectomy (PRK), involves the use of a nonthermal, high energy, pulsed laser to effectively sculpt the cornea into an ideal shape. For details, see, Marshall et al. "Photablative Reprofiling of the Cornea using an Excimer Laser: Photorefractive Keratectomy," Vol. 1, Lasers in Ophthalmology, pp. 21-48 (1986); and Tuft et al. "Stromal Remodelling Following Photorefractive Keratectomy," Vol. 1, Lasers in Ophthalmology, pp. 177-183 (1987), herein incorporated by reference.
The cornea of the eye is composed of a set of distinct layers: the outer epithelium, an anterior elastic lamina known as "Bowman's membrane," the cornea proper (or "stroma"), a posterior elastic lamina known as "Descemet's membrane", and the inner endothelium. The stroma is fibrous and transparent and constitutes the major portion of the cornea. Bowman's membrane, which forms the outer elastic lamina, is a rigid fibrillar structure not tending to cut or fracture, while Descemet's membrane, which forms the inner elastic lamina, is very brittle but elastic and has a tendency to curl. Together, the Bowman's and Descemet's membranes impart the necessary curvature to the stromal tissue.
Photorefractive keratectomy involves the use of a laser photoablation apparatus in which the size of the exposed area on the surface of the cornea is varied to control the reprofiling operation. In one embodiment, a beam-shaping stop or window is moved axially along the beam to increase or decrease the region of cornea on which the laser radiation is incident. By progressively varying the size of the exposed region, a desired photoablation profile is established on the surface. For further details on this technique, see U.S. Pat. No. 4,941,093 issued to Marshall et al. on Jul. 10, 1990, herein incorporated by reference.
Another new technique for corneal reshaping involves the use of a laser photoablation apparatus in which a beam-shaping mask is disposed between the laser and the surface. In one embodiment, the mask provides a predefined profile of resistance to erosion by laser radiation whereby a portion of the laser radiation is selectively absorbed and another portion is transmitted to the surface in accordance with the mask profile. For further disclosure of such masking techniques, see U.S. Pat. No. 4,856,513 issued to Muller on Aug. 15, 1989; U.S. Pat. No. 4,994,058 issued to Raven et al. on Feb. 19, 1991; and U.S. Pat. No. 5,019,074 issued to Muller on May 28, 1991, also incorporated herein by reference.
It is an object of the present invention to provide methods whereby laser surgical techniques can be applied to the eye to provide a correction for presbyopia.